In a conventional Package-on-Package (PoP) process, a top package is bonded to a bottom package. The top package and the bottom package are pre-formed before bonded to each other. The top package and the bottom package may also have device dies therein. By adopting the PoP process, the integration level of the packages may be increased.
In an existing PoP process, the formation of the bottom package includes bonding a device die to a package substrate. A molding compound is then molded on the package substrate, wherein the device die is molded in the molding compound. The package substrate further includes solder balls formed thereon, wherein the solder balls and the device die are on a same side of the package substrate. The solder balls are used for bonding the top package to the bottom package. Accordingly, to allow an adequate space for the device die, the heights of the solder balls need to be greater than the thickness of the device die, so that the top portions of the solder balls may protrude higher than the top surface of the device die, and higher than the top surface of the molding compound. Accordingly, the sizes of the solder balls are also large, and the number of the solder balls that can be used in a PoP structure is limited. In addition, with the large solder balls, the distance between neighboring solder balls also needs to be increased to prevent bridging.
Furthermore, the top package may also include solder regions formed thereon in order to connect to the solder balls in the bottom package. The large size of the solder balls and the additional solder from the top package makes it very difficult to prevent the bridging of solder balls while maintain the pitches of the solder balls to be small at the same time.